The present invention relates to an automatic transmission apparatus for vehicle, and in particular, to an automatic transmission apparatus for a vehicle equipped with a continuously variable transmission mechanism of the belt type which is a mechanical, continuously variable transmission apparatus.
An automobile using an internal combustion engine, such as a gasoline engine, as the prime mover has a transmission mechanism in its turning effort transfer system. In recent years, so-called automatic transmission apparatuses have been increasingly used in order to reduce troublesomeness caused by manual manipulation of the transmission mechanism.
A conventional automatic transmission apparatus mainly uses a hydraulic torque converter. A continuously variable mechanical transmission allowing continuously variable transmission operation within a preset value range was recently adopted as described in JP-A-62-53243, for example.
In a continuously variable transmission mechanism of the belt type used in the apparatus disclosed in JP-A-62-53243, a V-belt is wound across opposed conical pulleys, and the width and the effective radius of the pulley are changed, with the transmission ratio being substantially continuously controlled.
In the above described continuously variable transmission belt type mechanism a hydraulic actuator is used as the drive means for changing the pulley width in some cases.
When the pulley width is controlled by a hydraulic actuator, however, it is necessary to install a seal for preventing oil leakage and to provide a control valve, resulting in a problem of complication of the entire continuously variable transmission belt type mechanism.
Further in recent years, it has been proposed to adopt an electrical actuator as the drive means in order to simplify the structure of the continuously variable transmission belt type mechanism.
In a vehicle such as an automobile, however, a large torque acts between the belt and the conical pulley at the time of starting.
In both a continuously variable transmission belt type mechanism using a hydraulic actuator and a continuously variable transmission belt done mechanism using an electric actuator, slip is caused between the pulley and the belt when the vehicle is started. The driving force is thus abruptly transferred to the belt when the rotation of the pulley has risen up to some degree, resulting in a problem of shock generation.